For well over seventy years, attempts have been made by several workers to investigate the alloys belonging to Fe—N compositions and to synthesize Fe16N2 both in bulk and in thin film forms. Lehrer first reported magnetic measurements on Fe—N compositions. [E. Lehrer, Z. Elektrochem., 36, 460 (1930)] Fe16N2 was first identified by Jack in 1950 as precipitates in a matrix of nitrogen martensite that was obtained by quenching nitrogen austenite. [K. H. Jack, Proc. Roy. Soc. (London), A208, 216 (1951)] The Fe16N2 was found to form as a thin film on glass substrates by Kim and Takahashi and later on In—Ga—As substrates by Komura and co-workers. [T. K. Kim and M. Takahashi, Appl. Phys. Lett., 20, 492 (1972), M. Komura et al. J. Appl. Phys., 67, 5126 (1990)]. Despite attempts to refine Jack's method by several workers such as Wallace et al., and by Coey et al., Fe16N2 phase could not be formed as pure bulk form; significant secondary phases such as α-FeN and γ-FeN always were present in their materials. [M. Q. Huang et al., J. Magn. Magn. Mater., 135, 226 (1994) and J. M. D. Coey et al., J. Phys. Condens. Matter, 6, L23 (1994)].
Soft magnetic materials are known for use in electromagnetic devices. Typically, the magnetic material used for a core in such devices is selected from a soft magnetic material of high magnetic permeability such as silicon steel, magnetic metals, amorphous iron-based ribbons, pure iron powder, iron-based powder compositions, soft magnetic ferrites and the like. For example, magnetic materials such as iron, sendust, ferrosilicon, permalloy, supermalloy, iron-aluminum alloys, iron-cobalt alloys, manganese-zinc ferrites and nickel-zinc ferrites are used. Materials having high saturation magnetization are preferred, such as iron or iron-based compositions.
High performance soft magnetic materials with a large magnetic induction also play a role in advanced device development. The use of materials having improved magnetic induction over existing materials can provide significant weight reduction. The U.S. Navy is presently pursuing electromagnetic aircraft launch systems (EMALS). Such a system offers higher launch energy capability, as well as substantial improvements in areas other than performance. These include increased controllability, reliability, and efficiency.
The present EMALS design centers around a linear motor. At present, the linear motor is built using commercially available soft steel. Such a soft steel typically exhibits a saturation magnetization of 200 to 205 emu/g. A linear motor built using an iron nitride powder described in this invention as a precursor offers a significant weight savings compared to the motor built using commercially available soft steel. Other applications for the improved iron nitride include light-weight inductors, motors, actuators, and other microelectronics devices.
Masada et al., US Patent Publication 2005/0123754, describe a process to prepare an iron nitride powder substantially comprised of Fe16N2 and having a particle size of 50 nm or less. The powder is used as a magnetic layer in high-density magnetic recording media. Other uses for the powder are not described.
Masada et al., US Patent Publication 2005/0208320, disclose an iron nitride powder comprised primarily of Fe16N2 and having an average particle diameter of 20 nm or less and a standard deviation of the particle diameter of 14 nm or less. The powder is prepared using goethite having Al in solid solution for the starting iron oxide material. The powder is described as useful for magnetic materials in high density magnetic recording media. No other applications are described.
Masada et al., US Patent Publication 2006/0131537, disclose an iron nitride powder for use in high-density magnetic recording media, the powder being comprised of Fe16N2 particles to which particles of Si, P and/or Ti are adhered, and having a specific C/Fe atomic ratio.
The Masada patents disclose the use of a fixed bed reactor to make the powder. Further, the Masada patents are concerned with increasing coercivity of the iron powder, not increasing the magnetization, and only describe the use of iron nitride powder for magnetic recording applications.